The discussion of the background of the invention herein is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge at the priority date of any of the claims.
An optical lens is typically made of plastic or glass material and generally has two opposing surfaces which co-operate with one another to provide a required corrective prescription. When the positioning or shape of one of these surfaces with respect to the other is inaccurate, optical errors can be created.
Manufacturing of an optical lens to the required prescription requirements typically includes machining the surface of a semi-finished lens or lens blank. Typically, a semi-finished lens has a finished surface, for example the front surface and an unfinished surface, for example the back surface. By machining the back surface of the lens to remove material, the required shape and positioning of the back surface with respect to the front surface for the desired corrective prescription can be generated.
During manufacturing of the lens it is important that the semi-finished lens is securely maintained in an accurate position in order to prevent the generation of optical errors. Therefore, the manufacturing method comprises a blocking step during which the semi-finished lens is blocked on a blocker.
During the blocking step, the semi-finished lens is maintained by blocking the finished surface of the lens on a blocking ring. Various materials may be employed to secure the semi-finished lens to the blocking ring. These materials include low temperature fusible metal alloys and thermoplastic materials.
The position of the optical lens on the blocking ring can be of great importance so as to assure an accurate position of the manufactured surface.
In particular when manufacturing an optical lens one may wish to control the prism of the manufactured optical lens. The control of such prism requires controlling accurately the position of the optical lens on the blocking ring.
Furthermore, recent designs of progressive ophthalmic lenses include a combination of two complex surfaces, for example two aspherical surfaces, such as two progressive or regressive surfaces. When combining two complex surfaces, the relative position of both surfaces is of great importance for the optical function of the manufactured ophthalmic lens. Therefore, the semi-finished lens member has to be blocked in an accurate position during the blocking step or at least one needs to be able to determine the blocking position.